The gasification of particulate coal in a fluidized bed is known, having been disclosed by Garbo in U.S. Pat. Nos. 2,683,657 and 2,729,597, and by Forney in U.S. Pat. No. 3,463,623. In these patents, the particulate coal, on entering the gasification zone, is heated very quickly to the gasification temperature. Such rapid heating of the coal causes a decrease in coal density and also the coal particles structure is weakened. This results in the breaking up of coal particles in the gasifier to form low density fines. The decrease in coal density is a result of the coal particles undergoing swelling during the rapid heating. Such swelling is particularly noted in caking type coals. The swelling of caking coals upon heating has been studied and is referred to in "Some Relations in Rank and Rate of Heating to Carbonization Properties of Coal", J. S. Dulhunty and B. L. Harrison, Fuel, 32 (1953) at pages 441 et seq, and further in "Coal Typology -- Chemistry -- Physics -- Constitution", D. W. Van Krevelen, Elsevier Publishing Company, New York 1961, at pages 263 et seq.
In U.S. Pat. No. 2,131,702 Berry disclosed a process for making solid carbonized fuel, such as briquettes, from ground coal and a binder material utilizing very slow heating of the briquettes in non-fluidized beds with a hot non-reactive flue gas. This procedure minimized swelling of the briquettes and thereby maintained adequate strength and smooth surface during devolatilization.
Because of their low density, the hot fine coal particles are difficult to retain in the reaction zone when it is operated under fluidizing conditions. The result has been that a significant portion of the coal particle fines are carried out of the gasifier in the product gas stream along with some tars that are evolved during coal heating and gasification. This combination may cause plugging difficulties in the effluent conduits, resulting in frequent shutdowns of the process. Use of a longer free-fall section for heating the coal particles above the gasification zone has not been successful in overcoming such swelling and carryover problems. Although Jequier (U.S. Pat. No. 2,772,954) has disclosed a process for gasification of various type coals in multiple-beds, he utilizes continuous transfer of the particulate coal from the upper to lower beds, which involves quite rapid heating of the incoming coal particles to the average temperature of the coal in each bed. Furthermore, he operates his top bed at a sufficiently high temperature (500.degree. C) to avoid the formation of tars therein.
The gasification of coal encounters added difficulties when caking type coals are used. The caking type coals tend to agglomerate as well as swell when subjected to the high temperatures employed in gasification, and cause handling problems.
Non-caking coals are of the anthracite and semianthracite types and caking or agglomerating coals are usually of the bituminous types described by the Committee on Chemical Utilization of Coal in "Chemistry of Coal Utilization", John Wiley & Sons, Inc., New York, Volume 1, 1945, pages 183 et seq and also by P. J. Wilson and J. H. Wells in "Coal, Coke and Coal Chemicals", Chemical Engineering Series, McGraw-Hill Book Company, Inc., New York 1950, pages 47 et seq.
Such problems in the gasification of various coals have made it difficult to develop a gasification process that can adequately handle the various types of coals, and especially the swelling type caking coals, to produce a clean fuel gas product.